LispKit Draw Turtle

Library (lispkit draw turtle) defines a simple "turtle graphics" API. The API provides functionality for creating turtles and for moving turtles on a plane generating drawings as a side-effect. A drawing is a data structure defined by library (lispkit draw).

A turtle is defined in terms of the following components:

• A position (x, y) defining the coordinates where the turtle is currently located within a coordinate system defined by parameters used to create the turtle via make-turtle
• A heading angle which defines the direction in degrees into which the turtle is moving
• A boolean flag pen down which, if set to #t, will make the turtle draw lines on the graphics plane when moving.
• A line width defining the width of lines drawn by the turtle
• A color defining the color of lines drawn by the turtle
• A drawing which records the moves of the turtle while the pen is down.

Turtles are mutable objects created via make-turtle. The functions listed below change the state of a turtle. In particular, they generate a drawing as a side-effect which can be accessed via turtle-drawing. For most functions, the turtle is an optional argument. If it is not provided, the function applies to the turtle provided by the current-turtle parammeter object.

current-turtle     [parameter object]

Defines the current turtle, which is used as a default by all functions for which the turtle argument is optional. If there is no current turtle, this parameter is set to #f.

(turtle? obj)     [procedure]

Returns #t if obj is a turtle. Otherwise, it returns #f.

(make-turtle x y scale)     [procedure]

Returns a new turtle object. x and y determine the "home point" of the turtle. This is equivalent to the zero point of the coordinate system in which the turtle navigates. scale is a scaling factor.

(turtle-drawing turtle)     [procedure]

Returns the drawing associated with the given turtle.

(pen-up)     [procedure]
(pen-up turtle)

Lifts turtle from the plane. If turtle is not provided, the turtle defined by current-turtle is used. Subsequent forward and backward operations don't lead to lines being drawn. Only the current coordinates are getting updated.

(pen-down)     [procedure]
(pen-down turtle)

Drops turtle onto the plane. If turtle is not provided, the turtle defined by current-turtle is used. Subsequent forward and backward operations will lead to lines being drawn.

(pen-color color)     [procedure]
(pen-color color turtle)

Sets the drawing color of turtle to color. If turtle is not provided, the turtle defined by current-turtle is used. color is a color object as defined by library (lispkit draw).

(pen-size size)     [procedure]
(pen-size size turtle)

Sets the pen size of turtle to size. If turtle is not provided, the turtle defined by current-turtle is used. The pen size corresponds to the width of lines drawn by forward and backward.

(home)     [procedure]
(home turtle)

Moves turtle to its home position. If turtle is not provided, the turtle defined by current-turtle is used.

(move x y)     [procedure]
(move x y turtle)

Moves turtle to the position described by the coordinates x and y. If turtle is not provided, the turtle defined by current-turtle is used.

(heading angle)     [procedure]
(heading angle turtle)

Sets the heading of turtle to angle. If turtle is not provided, the turtle defined by current-turtle is used. angle is expressed in terms of degrees.

(turn angle)     [procedure]
(turn angle turtle)

Adjusts the heading of turtle by angle degrees. If turtle is not provided, the turtle defined by current-turtle is used.

(right angle)     [procedure]
(right angle turtle)

Adjusts the heading of turtle by angle degrees. If turtle is not provided, the turtle defined by current-turtle is used.

(left angle)     [procedure]
(left angle turtle)

Adjusts the heading of turtle by -angle degrees. If turtle is not provided, the turtle defined by current-turtle is used.

(forward distance)     [procedure]
(forward distance turtle)

Moves turtle forward by distance units drawing a line if the pen is down. If turtle is not provided, the turtle defined by current-turtle is used.

(backward distance)     [procedure]
(backward distance turtle)

Moves turtle backward by distance units drawing a line if the pen is down. If turtle is not provided, the turtle defined by current-turtle is used.